Contaminated completeness mask (left panel) and percentage difference compared to the original completeness mask (right panel). The contamination is introduced by multiplying the original mask by a factor of (1 − 5F) where F is a foreground template, in this case, the dust extinction map downgraded to the angular resolution of the colour indexing map depicted in Fig. 1. The factor α = 5 is chosen such that the mean contamination is 15%, an arbitrary choice to ensure that the contaminations are significant in the completeness mask. The difference between the original and contaminated masks shows that the effect is stronger on the edges of the survey.